There is a known phenomenon in which a person becomes incapable of recognizing the flicker of a TV screens or the like that was visible when he/she was in a healthy condition. When light is alternately turned ON and OFF at a high speed, the human eye is incapable of perceiving the switching of the light, and instead perceives the light as being continuously ON (in the case of a display device, they perceive the image as being constantly displayed). When the frequency for turning the light ON and OFF is gradually decreased, the flicker of the light becomes visible at a certain frequency. This frequency is called the flicker value (Critical flicker fusion rate: CFF). It is known that although the flicker value is relatively high for persons who are in a generally healthy condition, the value decreases with the increase of fatigue and, in particular, mental fatigue. Using this phenomenon, Simon & Enzer et al. (1941) proposed a flicker value examination method more than about 65 years ago. This method evaluates the degree of mental fatigue based on the decrease in the flicker value. Since then, the flicker value has been used as an index of mental fatigue or level of arousal in the field of ergonomics and occupational health.
This flicker value examination method is well recognized as combining numerous excellent properties, such as (1) the confirmation of (constant) changes in the measurement value with the application of a continuous fatigue load, (2) a constant correlation between changes in the measurement value and changes in the condition of activity (e.g., working efficiency), (3) a small fluctuation among measured values, and (4) a close relationship with the activity level of the cerebral cortex. Despite such advantages, the flicker value examination method did not become widely used due to the huge size of its measurement device.
The flicker value measurement method is disclosed, for example, in Patent Literatures 1 and 2 below. Patent Literature 1 discloses a flicker sensitivity distribution measurement device comprising a display for emitting a blinking light target. This apparatus uses blinking frequencies of 5 Hz, 10 Hz, 20 Hz, and 30 Hz.
Patent Literature 2 discloses a system comprising a blinking light emission apparatus and a computer terminal. In the system, the blinking light emission apparatus that is controlled by a computer via a communication cable displays flicker stimulation; and the computer records the test subject's button-pressing operations, each of which indicates the perception of flicker. The recorded data is then compared with previously measured data to evaluate the degree of fatigue.
Further, it is reported that the flicker value is proportional to the modulation (change) amount of the light stimulation. For example, it is reported that the flicker value is greater in long-duration flicker stimulation than in short-duration flicker stimulation, and that the flicker value is greater in high-luminance flicker stimulation than in low-luminance flicker stimulation. Moreover, it is also reported that the flicker value changes depending on the size or color of the stimulation object.